It is a known method to monitor an operation or a manufacturing process by means of a plurality of computers and a plurality of monitors. Take an example, in an automatic manufacturing process, it is usually provided with a plurality of computers and a plurality of monitors at some monitoring points, and then collecting the video signals outputted from said monitors by a management computer, so that a supervisor can monitor and control the whole manufacturing process.
The disadvantage of the above known method can be described in FIG. 1. As shown in FIG. 1, the circuit diagram of a known device for monitoring video signals of a plurality of computers is indicated by the symbol 10, wherein four sets of red, green, and blue video signals of four computer terminals, i.e. (Ra, Ga, Ba), (Rb, Gb, Bb), (Rc, Gc, Bc), (Rd, Gd, Bd), are shown. The red video signals Ra, Rb, Rc, Rd are inputted into a first set of relays 11, the green video signals Ga, Gb, Gc, Gd are inputted into a second set of relays 12, and the blue video signals Ba, Bb, Bc, Bd are inputted into a third set of relays 13.
If the supervisor will monitor video signals of the second computer terminal, he can input two video selecting signals S11, S12 to a decoder 16 for generating four control signals Ca, Cb, Cc, Cd, wherein only Cb can make the corresponding relay in each set to be conducting, while other signals Ca, Cc, Cd will make other relays in each set open, thus the video signals Rb, Gb, Bb of the second computer terminal will pass through the corresponding relays and to be inputted into a connector 14. Additionally, said two video selecting signals S11, S12 are also inputted into a multiplexor 17 (which are inputted with the four sets of vertical and horizontal synchronous signals (Va, Ha), (Vb, Hb), (Vc, Hc), (Vd, Hd) of the four computer terminals) for selecting the vertical and horizontal synchronous signals Vb, Hb of the second computer terminal, and being inputted into the connector 14. The connector 14 will then send the video signals Rb, Gb, Bb and the horizontal and vertical sync signals Vb, Hb to a monitor 15 for being monitored by the supervisor.
The disadvantage of the above known method is in that the three sets of relays are analog mechanical devices, and there is no circuit for signal compensation, so the inputted video signals will be seriously attenuated when the distance is long or the frequency is high. Therefore, the distance between the monitor 15 and the four computer terminals are limited, and the frequency bandwidth of the video signals are also limited.
In order to improve the disadvantage of the circuit in FIG. 1, another three sets of analog IC switches 21, 22, 23 (as shown in FIG. 2) comprising transistor pairs (i.e. NPN transistor interconnected with PNP transistor, or PMOS transistor interconnected with NMOS transistor) are used to replace the three sets of relays 11, 12, 13 in FIG. 1 for being inputted with AC type of video signals, and an operational amplifier IC 24 (comprising three sets of operational amplifier 241,242, 243) are added before the connector 25 to amplify the selected video signals for compensation.
In FIG. 2, four sets of red, green, and blue video signals of four computer terminals, i.e. (Ra, Ga, Ba), (Rb, Gb, Bb), (Rc, Gc, Bc), (Rd, Gd, Bd), are shown. The red video signals Ra, Rb, Rc, Rd are inputted into a first set of transistor pair analog IC switch 21, the green video signals Ga, Gb, Gc, Gd are inputted into a second set of transistor pair analog IC switch 22, and the blue video signals Ba, Bb, Bc, Bd are inputted into a third set of transistor pair analog IC switch 23.
If the supervisor will monitor video signals of the second computer terminal, he can input two video selecting signals S21, S22 to a decoder 27 for generating four control signals Ca, Cb, Cc, Cd, wherein only Cb can make the corresponding transistor pair in each set to be conducting, while other signals Ca, Cc, Cd will make other transistor pairs in each set open, thus the video signals Rb, Gb, Bb of the second computer terminal will pass through the corresponding transistor pairs and being amplified by the first, the second, and the third video operational amplifiers 241, 242, 243 to be inputted into a connector 25.
Additionally, said two video selecting signals S21, S22 are also inputted into a multiplexor 28 (which are inputted with the four sets of vertical and horizontal synchronous signals (Va, Ha), (Vb, Hb), (Vc, Hc), (Vd, Hd) of the four computer terminals) for selecting the vertical and horizontal synchronous signals Vb, Hb of the second computer terminal, and being inputted into the connector 25. The connector 25 will then send the video signals Rb, Gb, Bb and the horizontal and vertical sync signals Vb, Hb to a monitor 26 for being monitored by the supervisor.
The disadvantage of the circuit in FIG. 2 is in that the transistor pair analog IC switches 21, 22, 23 will make the circuit design more complicated and more expensive. Additionally, since the switches 21, 22, 23 and the operational amplifiers 241, 242, 243 are IC packaged, the frequency bandwidth are therefore limited, video signals higher than 100 MHz will be attenuated abruptly, this limitation apparently makes the circuit in FIG. 2 unadaptable to high frequency video signals currently used. If we try to use high frequency IC switches and operational amplifiers, the cost will be raised dramatically. Furthermore, the video operational amplifiers 241, 242, 243 need a negative voltage besides a positive voltage, so the circuit are complicated and will increase more noises.